Not applicable.
The present invention relates to a process for the preparation of hydrocarbons from synthesis gas, i.e., a mixture of carbon monoxide and hydrogen, typically labeled the Fischer-Tropsch process. More particularly, this invention relates to the use of acrylate polymer matrix structures for catalysts for the Fischer-Tropsch process. Still more particularly, the present invention relates to Fischer-Tropsch catalysts formed by polymerizing an acrylate with a catalytically active metal.
Large quantities of methane, the main component of natural gas, are available in many areas of the world, and natural gas is predicted to outlast oil reserves by a significant margin. However, most natural gas is situated in areas that are geographically remote from population and industrial centers. The costs of compression, transportation, and storage make its use economically unattractive. To improve the economics of natural gas use, much research has focused on the use of methane as a starting material for the production of higher hydrocarbons and hydrocarbon liquids, which are more easily transported and thus more economical. The conversion of methane to hydrocarbons is typically carried out in two steps. In the first step, methane is converted into a mixture of carbon monoxide and hydrogen (i.e., synthesis gas or syngas). In a second step, the syngas is converted into hydrocarbons.
This second step, the preparation of hydrocarbons from synthesis gas, is well known in the art and is usually referred to as Fischer-Tropsch synthesis, the Fischer-Tropsch process, or Fischer-Tropsch reaction(s). Fischer-Tropsch synthesis generally entails contacting a stream of synthesis gas with a catalyst under temperature and pressure conditions that allow the synthesis gas to react and form hydrocarbons.
More specifically, the Fischer-Tropsch reaction is the catalytic hydrogenation of carbon monoxide to produce any of a variety of products ranging from methane to higher alkanes and aliphatic alcohols. Research continues on the development of more efficient Fischer-Tropsch catalyst systems and reaction systems that increase the selectivity for high-value hydrocarbons in the Fischer-Tropsch product stream.
There are continuing efforts to find catalysts that are more effective at producing these desired products. Product distribution, product selectivity, and reactor productivity depend heavily on the type and structure of the catalyst and on the reactor type and operating conditions. It is particularly desirable to maximize the production of high-value liquid hydrocarbons, such as hydrocarbons with five or more carbon atoms per hydrocarbon chain (C5+).
Catalyst supports for catalysts used in Fischer-Tropsch synthesis of hydrocarbons have typically been oxides (e.g., silica, alumina, titania, zirconia or mixtures thereof, such as silica-alumina). The products prepared by using these catalysts usually have a very wide range of molecular weights. It has been asserted that the Fischer-Tropsch synthesis reaction is only weakly dependent on the chemical identity of the metal oxide support (see E. Iglesia et al. 1993, In: xe2x80x9cComputer-Aided Design of Catalysts,xe2x80x9d ed. E. R. Becker et al., p. 215, New York, Marcel Dekker, Inc.). Nevertheless, because it continues to be desirable to improve the activity of Fischer-Tropsch catalysts, other types of catalyst supports have been investigated.
The use of divinylbenzene cross-linked polystyrene as a support for Fischer-Tropsch catalysts is disclosed in U.S. Pat. No. 4,292,415 and U.S. Pat. No. 4,725,568. Similarly, U.S. Pat. No. 4,230,633 discloses polymer supported metal complexes wherein the ligand is a cycloalkadienyl radical with metals from Group VIII of the Periodic Table. This patent relates to the conversion of carbon monoxide and hydrogen to hydrocarbons in a liquid reaction medium. Nevertheless, despite the research in this field, there is still a desire to identify new, more effective catalysts. In particular, catalysts that provide high C5+ and C11+ productivities are desired.
The present invention provides a catalyst system that is effective for producing C5+ and C11+ hydrocarbons. In accordance with a preferred embodiment, the present catalyst comprises (1) cobalt and at least one other metal selected from the group consisting of silver, iron, zinc and zirconium and (2) a matrix structure comprising a polymer selected from the group consisting of polyacrylates and polymethacrylates. The catalyst so formed is preferably treated with hydrogen at a temperature of at least 400xc2x0 C. prior to use. Catalyst systems constructed in accordance with the invention compare favorably to previously known catalysts in activity and durability.
The present invention further comprises a process for using the present catalyst system to produce hydrocarbons. The process comprises contacting a feed stream comprising hydrogen and carbon monoxide with the present catalyst system in a reaction zone maintained at conditions that are effective to produce an effluent stream comprising hydrocarbons.
The present catalyst system comprises a catalytic composition integrated into a polymeric matrix structure. The catalytic composition preferably comprises at least one Group VIII metal, namely iron, nickel, cobalt, rhenium, ruthenium, chromium, and iridium or mixtures thereof, and at least one other metal selected from the group consisting of silver, iron, zinc and zirconium. Of these, cobalt/silver is most preferred. The catalytic composition may further include one or more promoters selected from the group consisting of alkali and alkaline earth metal in free or combined form, boron, and mixtures thereof.
The polyacrylate and polymethacrylate matrix structures used in the process of this invention can be prepared by the polymerization of metal acrylates and/or metal methacrylates. Several alternative techniques are suitable for achieving the desired polymerization.
Formation of Cobalt acrylate
In one preferred technique, the present catalysts are formed by mixing the desired monomer with a salt of the desired metal catalyst and an initiator in a solvent. Polymerization occurs with mixing, producing a polymerized mass. The metal salt is preferably suspended in water and reacted with the acrylic acid or methacrylic acid at a temperature between about 40 and about 60xc2x0 C. By way of example only, suitable metal salts include but are not limited to: basic cobalt carbonate, silver carbonate or silver oxide, iron carbonate or iron oxide, zinc carbonate or zinc oxide and zirconium carbonate or zirconium hydroxide. The reactions are carried out for about 5 hours with gradual addition of the acrylic acid or methacrylic and constant stirring. The solid product is extracted with ethanol and the extract is filtered and evaporated to dryness.
Technique I
The present catalysts can be prepared by dissolving a metal acrylate or metal methacrylate, such as cobalt acrylate, cobalt carbonate, or cobalt methacrylate (described above) and at least one other metal acrylate or methacrylate in methanol, ethanol or another suitable alcohol. The alcohol can contain as much as 25 wt % water. The polymerization is preferably carried out at reflux temperature using 2,2xe2x80x2-azobisisobutyronitrile as an initiator. The polymerization reaction mixture is refluxed for at least 3 hours with constant stirring. The reaction product is filtered off, washed several times with ethanol and then dried under vacuum at 40 to 60xc2x0 C.
Technique II
Alternatively, the present catalysts can be prepared by dissolving polyacrylic acid or polymethacrylic acid in water, followed by addition of the cobalt and at least one metal acrylate or metal methacrylate to the polymer solution under stirring. The solution gels and is evaporated to dryness.
Catalyst
The metal-containing polyacrylate or polymethacrylate catalysts produced by any of the preceding techniques are preferably reduced with hydrogen at a temperature of at least 400xc2x0 C. before use as a Fischer-Tropsch catalyst. Treatment with hydrogen activates the catalyst. Catalysts produced in this manner do not require supports.
Operation
The present catalysts are preferably used in a Fischer-Tropsch reactor charged with feed gases comprising hydrogen or a hydrogen source and carbon monoxide. H2/CO mixtures suitable as a feedstock for conversion to hydrocarbons according to the process of this invention can be obtained from light hydrocarbons such as methane by means of steam reforming or partial oxidation. The hydrogen is preferably provided by free hydrogen, although some Fischer-Tropsch catalysts have sufficient water gas shift activity to convert some water to hydrogen for use in the Fischer-Tropsch process. It is preferred that the mole ratio of hydrogen to carbon monoxide in the feed be greater than 0.5:1 (e.g., from about 0.67:1 to 2.5:1). The feed gas may also contain carbon dioxide or other compounds that are inert under Fischer-Tropsch reaction conditions, including but not limited to nitrogen, argon, or light hydrocarbons. The feed gas stream should contain a low concentration of compounds or elements which have a deleterious effect on the catalyst. The feed gas may need to be treated to ensure low concentrations of sulfur or nitrogen compounds such as hydrogen sulfide, ammonia and carbonyl sulfides.
The feed gas is contacted with the catalyst in a reaction zone. Mechanical arrangements of conventional design may be employed as the reaction zone. For example, fixed bed, slurry phase, slurry bubble column, fluidized bed, or ebulliating bed reactors. Accordingly, the size of the catalyst particles may vary depending on the reactor in which they are to be used.
The process is typically run in a continuous mode. In this mode, typically, the gas hourly space velocity through the reaction zone may range from about 100 volumes/hour/volume catalyst (v/hr/v) to about 10,000 v/hr/v, preferably from about 300 v/hr/v to about 2,000 v/hr/v. The reaction zone temperature is typically in the range from about 160xc2x0 C. to about 300xc2x0 C. Preferably, the reaction zone is operated at conversion promoting conditions at temperatures from about 190xc2x0 C. to about 260xc2x0 C. The reaction zone pressure is typically in the range of about 80 psig (653 kPa) to about 1000 psig (6994 kPa), preferably, from 80 psig (653 kPa) to about 600 psig (4237 kPa), more preferably, from about 140 psig (1066 kPa) to about 400 psig (2858 kPa).
The reaction products will have a large range of molecular weights. The present catalysts are particularly useful for making hydrocarbons having five or more carbon atoms, especially when the above-referenced space velocity, temperature and pressure ranges are employed.
The wide range of hydrocarbon species produced in the reaction zone will typically result in liquid phase products at the reaction zone operating conditions. Therefore, the effluent stream of the reaction zone will often be a mixed phase stream. The effluent stream of the reaction zone may be cooled to effect the condensation of additional amounts of hydrocarbons and passed into a vapor-liquid separation zone. The vapor phase material may be passed into a second stage of cooling for recovery of additional hydrocarbons. The liquid phase material from the initial vapor-liquid separation zone together with any liquid from a subsequent separation zone may be fed into a fractionation column. Typically, a stripping column is employed first to remove light hydrocarbons such as propane and butane. The remaining hydrocarbons may be passed into a fractionation column wherein they are separated by boiling point range into products such as naphtha, kerosene and fuel oils. Hydrocarbons recovered from the reaction zone and having a boiling point above that of the desired products may be passed into conventional processing equipment such as a hydrocracking zone in order to reduce their molecular weight. The gas phase recovered from the reactor zone effluent stream after hydrocarbon recovery may be partially recycled if it contains a sufficient quantity of hydrogen and/or carbon monoxide.
Without further elaboration, it is believed that one skilled in the art can, using the description herein, utilize the present invention to its fullest extent. The following embodiments are to be construed as illustrative, and not as constraining the remainder of the disclosure in any way whatsoever.